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Hubble sees the fireball from a kilonova

NASA’s Hubble Space Telescope has detected a new kind of stellar blast called a kilonova that’s about 1,000 times brighter than a regular nova.

A kilonova happens when a pair of compact objects such as neutron stars crash together. Hubble observed the fading fireball from a kilonova last month, following a short gamma ray burst (GRB) in a galaxy almost 4 billion light-years from Earth.

In the image at left, the galaxy in the center produced the gamma-ray burst, designated GRB 130603B. The galaxy resides almost 4 billion light-years away. A probe of the galaxy with Hubble’s Wide Field Camera 3 on June 13, 2013, revealed a glow in near-infrared light at the source of the gamma-ray burst, shown in the image at top, right. When Hubble observed the same location on July 3, the source had faded, shown in the image at below, right. The fading glow provided key evidence that it was the decaying fireball of a new type of stellar blast called a kilonova. Image credit: NASA, ESA, N. Tanvir (University of Leicester), A. Fruchter (STScI), and A. Levan (University of Warwick)

“This observation finally solves the mystery of short gamma ray bursts,” says Nial Tanvir of the University of Leicester in the United Kingdom, who led a team of researchers conducting this research.

Gamma ray bursts are flashes of intense high-energy radiation that appear from random directions in space. They come in two flavors–long and short. “Many astronomers, including our group, have already provided a great deal of evidence that long-duration gamma ray bursts (those lasting more than two seconds) are produced by the collapse of extremely massive stars,” explains Tanvir.

The short bursts, however, were more mysterious.

We only had weak circumstantial evidence that short bursts [might be] produced by the merger of compact objects,” he adds. “This result now appears to provide definitive proof.”

Astrophysicists have predicted short-duration GRBs are created when a pair of super-dense neutron stars in a binary system spiral together. This event happens as the system emits gravitational radiation, creating tiny waves in the fabric of space-time. The energy dissipated by the waves causes the two stars to sweep closer together. In the final milliseconds before the explosion, the two stars merge into a death spiral that kicks out highly radioactive material. This material heats up and expands, emitting a burst of light.

This sequence illustrates a model for the formation of a short-duration gamma-ray burst.1. A pair of neutron stars in a binary system spiral together. Orbital momentum is dissipated through the release of gravity waves, which are tiny ripples in the fabric of space-time.2. In the final milliseconds, as the two objects merge, they kick out highly radioactive material. This material heats up and expands, emitting a burst of light called a kilonova. An accompanying gamma-ray burst lasts just one-tenth of a second, but is 100 billion times brighter than the kilonova flash.3. The fading fireball blocks visible light but radiates in infrared light.4. A remnant disk of debris surrounds the merged object, which may have collapsed to form a black hole.Image credit: NASA, ESA, and A. Field (STScI)

The resulting “kilonova” is about 1,000 times brighter than a regular nova, which is caused by the eruption of a white dwarf.

In a recent science paper Jennifer Barnes and Daniel Kasen of the University of California at Berkeley and the Lawrence Berkeley National Laboratory presented new calculations predicting how kilonovas should look. They predicted the same hot plasma producing the radiation also will block the visible light, causing the gusher of energy from the kilonova to flood out in near-infrared light over several days.

An unexpected opportunity to test this model came June 3 when NASA’ s Swift space telescope picked up the extremely bright gamma ray burst, cataloged as GRB 130603B. Although the initial blast of gamma rays lasted just one-tenth of a second, it was roughly 100 billion times brighter than the subsequent kilonova flash.

From June 12-13, Hubble searched the location of the initial burst, spotting a faint red object. An independent analysis of the data from another research team confirmed the detection. Subsequent Hubble observations on July 3 revealed the source had faded away, therefore providing the key evidence the infrared glow was from an explosion accompanying the merger of two objects.

The team’s results appeared Aug. 3 in a special online publication of the journal Nature.